Golf club head

ABSTRACT

A golf club having an increased moment of inertia and improved ball-hitting directionality is provided. A metal hollow golf club head comprises: a face portion; a crown portion; and a sole portion, and when the golf club has a lie angle of 60° with its club head volume being within 470 cm 3 , a moment of inertia about the axial line centered on the plumb line passing through the golf club head center of gravity is 5000 to 6000 g-cm 2 . In order to increase the moment of inertia, the thickness of the center portion of the crown portion is reduced by chemical etching, and a mass, including the portion of mass reduction, is positioned in the sole portion on the side of the toe portion; moreover, the separation distance from the center of gravity to the mass is increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of Non Provisional Application No. 11/937,868 filed on Nov. 9, 2007, which claims priority to Japanese Application No. 2006-305003, filed on Nov. 10, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a golf club head with improved ball-hitting directionality, and more specifically, relates to a golf club head having a large moment of inertia and improved stability of direction of a hit ball.

2. Description of the Related Art

Various improvements have been made to golf clubs, to extend flight distances, and to enable stable hitting of the ball. Flight distances directly affect scores, and so through improvements so as to broaden the effective range (“sweet” area) of the striking point on a golf club head (hereafter also simply called a “head”), improvements in the position of the effective area, improvements in the material of the face surface, and similar, the probabilistic ball flight distance has been extended; consequently scores have improved, and players using such heads have found them beneficial. Moreover, in order to improve scores, golf clubs have been sought for which the direction of ball-hitting is stably determined even when there is deviation of the striking point. For this reason, normally the moment of inertia must be made large. In particular, the lateral moment of inertia is an important factor determining the direction of the ball.

This is because, if the moment of inertia is made large, when the striking position at which the ball is struck is shifted, so that for example the ball is struck on the toe side of the golf club head, the club does not readily bend. That is, if the moment of inertia of the golf club head is made large, then as explained above, even when the ball is hit off-center there is little shake of the head, and the ball is driven in a comparatively straight direction. Hence the average flight distance is extended, and as a result scores are improved.

The wood material (persimmon) of the woods used from long ago had a tendency to cause the head to rotate easily when the ball was stuck; but modern woods, made of metal and with a hollow interior, have a larger moment of inertia compared with wooden woods, so that there is little rotation and no similar tendency, so that at present such clubs are used by many players and have become the mainstream. These hollow metal-type woods have grown in volume, but current rules stipulate a maximum volume of 460 cm³ (with a tolerance of 10 cm³).

There is a trend toward larger heads, but the masses of the constituent parts of a head adds up, and the swing balance, which is a criterion for ease of swinging of a club, becomes heavy. As a result, head masses have in the prior art been no greater than approximately 210 g. That is, given the configuration of the prior art, although heads have tended to increase in size the total mass has been limited, and so excess mass to control the position of the center of gravity, or in other words, excess mass to increase the moment of inertia has in the prior art been limited to approximately 10 g, due to the constraint that the total mass should not be increased.

According to R&A rules, the pendulum test method is adopted to measure the restitution coefficient of the face surface. This testing method entails fixing the club, causing a steel sphere to collide with the face surface, and measuring the contact time; the contact time is called the characteristic time, and the rule limits this characteristic time to 257 μs (microseconds) or less (including a tolerance of 18 μs). In order to keep this characteristic time at or less than the time stipulated by the rules, the thickness of the face portion sheet tends to become thick, but there is a limit to the extent to which the face portion mass can be reduced. Further, the hosel portion connected to the shaft is positioned on one end of the face portion, and the weight of this portion is also relatively great.

Further, the above-described rules also impose various constraints on external dimensions, such as that the length from the heel portion to the toe portion must not be longer than the length from the face portion to the rear surface; that the length from the heel to the toe must be 127 mm (5 inches) or less; and that the length from the sole to the crown must be 71.12 mm (2.8 inches) or less. And, there is the further constraint that the volume must be 470 cm³ (including a tolerance of 10 cm³) or less. Given these constraints as well, the center of gravity position cannot be located in a position so as to increase the “sweet” area, and moreover the overall head mass becomes large. Due to such constraints, it is extremely difficult to increase the weight or other excess mass so as to increase the moment of inertia.

Despite such engineering difficulties, various proposals to increase the moment of inertia have been made. For example, heads are known in which mass is distributed in at least one direction among the three major inertial axes in orthogonal coordinates passing through the center of gravity, or in sites in proximity thereto, or with masses distributed in such a manner (see for example Japanese Patent Laid-open No. 5-57034). And, clubs are known in which the golf club head comprises metal material members and fiber-reinforced resin members, with the head bonded together by an adhesive of thickness 0.05 to 1 mm (see for example Japanese Patent Laid-open No. 2003-320060). Also, technology is known in which an aperture portion is provided in the crown portion, and a fiber-reinforced resin with specific gravity smaller than metal materials is used in this aperture portion, to improve the ball-hitting directionality (see for example Japanese Patent Laid-open No. 2005-278838).

Thus various measures have been taken to extend the flight distance of golf clubs, but at present club performance remains not entirely satisfactory. While the technologies described above have represented partial improvements, problems remain, and there is still room for improvement. Metal hollow golf club heads tend to increase in size, as described above, and if the moment of inertia is increased, the volume tends to increase as well; if the volume is increased while making efforts to limit mass, strength-related problems arise; and so there have been limits to the methods employed in the prior art. When for example using fiber-reinforced resins as described above, not only do strength-related limits appear, but there are the problems of unsatisfactory ball-hitting sounds and resistance to damage. On the other hand, insofar as is known by these inventors, there exist no golf clubs in the prior art, primarily comprising metal members, with a moment of inertia in the range 5000 to 6000 g-cm².

This invention was devised in order to resolve the above-described problems of the prior art, and attains the following objects.

SUMMARY OF THE INVENTION

An object of the invention is to provide a golf club with an increased moment of inertia of a metal hollow golf club head with large volume, and with improved ball-hitting directionality.

A further object of the invention is to provide a golf club with an increased moment of inertia of a metal hollow golf club head with large volume without increasing the head mass, and with improved ball-hitting directionality.

In order to attain the above objects, the following means are employed.

The golf club of Invention 1 is a golf club, comprising: a face portion positioned on the front surface of a metal hollow golf club head and having a striking face to strike a golf ball; a crown portion forming an upper surface of the club, and a sole portion forming a lower surface of the club; wherein

a mass of the metal hollow golf club head is 210 g or less,

characteristic time (CT value) of the metal hollow golf club head, relating to a restitution characteristic, is 257 μs or less,

the volume of the metal hollow golf club head is 400 to 470 cm³ when a lie angle of the metal hollow golf club head is 60 degrees,

the metal is made of titanium alloy entirely and formed through welding to be an integrated body

the crown portion and sole portion constituting the body has an area of thick portion and an area of thinned portion, the area of the thinned portion amounts to more than 60% in both of the crown portion and the sole portion, and the thickness difference between the thinned portion and the thick portion is more than 0.2 mm and the removed mass amounts to more than 10 g in both of the crown portion and the sole portion,

a weight is located and fixed at a position of a rotation radius most distant from the plumb line and at toe-side and back-side sites of the crown portion and/or the sole portion,

the metal hollow golf club head has:

a length (X) from a heel portion to a toe portion longer than a length (Y) from a face portion to a rear surface;

a length from a heel to a toe of 127 mm (5 inches) or less;

a length from a sole to a crown of 71.12 mm (2.8 inches) or less; and

a moment of inertia (MOI) calculated using the following approximating equation (1):

MOI=(aY ² +bY+c)*(dX+e)/f   (1)

where, in the approximating equation (1), the second order equation of Y is determined by approximating the MOI value with a second order equation of variable Y as a head width under a constant value X as a toe-heel length and the constants a, b and c are coefficients of the terms in the second order equation respectively, the first order equation of X is determined by approximating the MOI value with a first order equation of a variable X as a toe-heel length under a constant value Y as a head width and d and e are constants of terms in the first order equation respectively, and f is a value of MOI with a certain selected value of X satisfying the required condition;

a position of the center of gravity (u, v), as seen from the plumb direction, exists within a range encompassed by the following two approximating equations:

v=gu ² +hu+i   (2)

v=ju ² +ku+1   (3)

where u is a position of center of gravity in the direction from the heel portion to the toe portion, v is a position of center of gravity in the direction from the face portion to the rear surface, u has an origin at the center of the distance from the heel portion to the toe portion and v has an origin in the face surface; and the approximating equation (2) is determined by approximating the relation between u and v with a second order equation when MOI is the maximum, g, h and i are constants of terms of the equation respectively, and the approximating equation (3) is determined by approximating the relation between u and v with a second order equation when MOI is 5000 gcm², j, k and l are constants of terms of the equation respectively, and

the moment of inertia about an axial line, which is the plumb line passing through the center of gravity of the metal hollow golf club head, is in a range of 5000 to 6000 g-cm².

The golf club of the Invention 2 is the golf club of Invention 1, characterized in that the metal hollow golf club head is formed by welding to join the face portion, the sole portion, the crown portion and a hosel portion to which the shaft is connected.

The golf club of the Invention 3 is the golf club of Invention 1, characterized in that the weight is 20 g or more.

The golf club of the Invention 4 is the golf club of Invention 1, characterized in that toe-side and back-side sites of the sole portion are formed in shapes protruding outward relative to the crown portion, and the weight is positioned in this protruding sole portion.

The golf club of the Invention 5 is the golf club of Invention 1, characterized in that the area of the thinned portion is larger in the crown portion than in the sole portion.

The golf club of the Invention 6 is the golf club of Invention 1, characterized in that the thinned portions in the crown portion and the sole portion are formed by chemical etching.

As explained in detail above, a golf club of this invention employs a hollow golf club head, the materials comprised by which are in essence all metals; under the constraints that the head volume be 470 cm³ or less (including a tolerance of 10 cm³), that the head mass be 210 g or less, and that the head characteristic time (CT value) related to the restitution characteristic be 257 μs or less (including a tolerance of 18 μs), a method and conditions were discovered for obtaining a golf club head for which the moment of inertia about the axial line centered on the plumb line passing through the center of gravity of the metal hollow golf club head is in the high range 5000 to 6000 g-cm². As a result, even when the striking point deviates from the center of the face portion, the direction in which the ball is hit is secured, and stable striking is possible compared with clubs of the prior art, so that as a result there is a strong possibility of improvement of a player's score. Further, because in essence the club head is of metal, satisfactory performance with respect to durability, ball-hitting sound, and other aspects can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view showing the overall configuration of a golf club;

FIG. 2 is a plane view of Aspect 1 of a driver club head;

FIG. 3 is a front view of Aspect 1 of a driver club head;

FIG. 4 is a side view of Aspect 1 of a driver club head;

FIG. 5 is a cross-sectional view along X-X in FIG. 4;

FIG. 6 is a plane view of Aspect 2 of a driver club head;

FIG. 7 is a plane view of Aspect 3 of a driver club head;

FIG. 8 is a front view of Aspect 3 of a driver club head;

FIG. 9 is a side view of Aspect 3 of a driver club head;

FIG. 10 arranges the external shapes of the heads 1 for which the moment of inertia MOI is calculated;

FIG. 11 shows the relation between head width and moment of inertia, when the toe-heel length is held constant;

FIG. 12 shows the relation between toe-heel length and moment of inertia when the head width is held constant;

FIG. 13 is a graph showing the relation between toe-heel length and head width for different moments of inertia;

FIG. 14A-E is an example in which the position of weight placement in a head 1 is changed; and,

FIG. 15 shows the relation between moment of inertia and position of center of gravity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspect 1

Aspect 1 of the invention is explained referring to the drawings. FIG. 1 is an external view of an entire golf club of this invention, and shows the driver club head having a metal hollow golf club head. A golf club of this invention employs a metal hollow golf club head; the driver club head (hereafter also called a “head”) of Aspect 1 similarly is a metal hollow golf club head. The basic construction of a driver club head is well-known, and a detailed explanation is omitted; however, in order to facilitate understanding of the invention, a summary explanation is given as follows.

A driver club head 1 of this invention has one end of the shaft A fixed. FIG. 2 to FIG. 4 show an aspect of a driver club head 1 of a metal hollow golf club of this invention. FIG. 2 to FIG. 6 show only the head portion; the shaft A and other members are not related to the gist of this invention, and so are omitted from the drawings.

FIG. 2 is a plane view of the head 1, FIG. 3 is a front view of the head 1, and FIG. 4 is a side view of the head 1. As shown in each of the drawings, the metal hollow driver club head 1 comprises a crown portion 2, which is the upper portion; a sole portion 3, which is the bottom portion; a face portion 4, which strikes the golf ball; a toe portion 5, which is the front portion of the head 1; a heel portion 6, which is the rear portion of the head; a back portion 10, positioned on the opposite side from the face portion 4, forming the rear portion of the head 1; and a hosel portion 7, which is a member which supports and fixes the driver club head 1 on the shaft A. Among these portions, the principal portions are the face portion 4, crown portion 2, sole portion 3, and hosel portion 7.

For manufacturing reasons, each of these portions comprises either one or a plurality of sheet members, which are joined together. In manufacturing each portion, sheet members are press-molded to the desired curved-surface shape, and are then integrated by welding or other means. Rolled members are particularly suitable for use as sheet members for purposes of controlling the sheet thickness. In this example, the body member comprising the head 1 is formed by combining four members, which are a face member; a sole member, comprising a toe portion 5, heel portion 6, and a portion of the back portion 10; a crown member, comprising a toe portion 5, heel portion 6, and a portion of the back portion 10; and a hosel member.

Each of the four members is formed by cutting the sheet members into prescribed shapes and then heating and pressing. The face member for example is heated to 400°, and the sole member, crown member, and other body member were heated to 900°. After pressing, burrs were cut away (trimming), and TIG welding was performed. TIG welding is a type of welding also called argon welding; a welding rod of the deposit metal itself is used, and argon gas is released from the periphery of a tungsten electrode, to shield the molten metal from the atmosphere during welding. Also, laser and plasma welding methods may be used, with fewer welding beads or thermal effects, making such methods more appropriate. Each of the members may also be manufactured by casting, forging, or another method.

In this Aspect 1, the metal material is a titanium alloy, the face member and sole member are opposed as members in manufacturing, and thereafter the hosel member is joined, and then the pressed crown member is bonded by TIG welding or similar. In this way, an integrated driver club head 1 is formed by welding. The face portion 4 has a minutely curvature surface, and is formed in a plate shape. The area of maximum restitution coefficient is the center portion which is the striking face, that is, the “sweet” area 9 near the center of gravity 8.

Normally, in order to send the golf ball a great distance, it is effective to strike the ball at this “sweet” area 9, corresponding to a position near the center of gravity 8; to this end, this area is made larger and the area with a high restitution coefficient is expanded, or, the restitution coefficient is set to a high value to enhance the restitution effect. As explained above, it is well known that if the restitution coefficient is high, the golf ball will travel a great distance, and this restitution coefficient constitutes an important parameter of golf club performance, so that as stated above measurement criteria are stipulated by the U.S. Golf Association (USGA), R&A rules, and other authorities. Titanium alloys often used in driver club heads 1 include β-type titanium alloys and α+β-type titanium alloys. These alloys have enhanced strength as well as excellent machinability, ductility, toughness, and strength, and are reliable alloys.

In this Aspect 1, the basic driver club head 1 comprised by such a golf club has improvements added to the crown portion 2 and sole portion 3 in particular, in order to increase the moment of inertia. The moment of inertia is represented by m×r²; hence it is clear to a practitioner of the art that it is sufficient to increase either m (the mass of the head 1) or r (the distance from the center of gravity). However, the mass of the general head overall is limited to approximately 210 g due to the fact that, as described above, if the head is made too heavy the swing balance suffers. In particular, due to the larger volumes of recent heads 1, the mass of the head 1 cannot be made too great.

Hence given these constraints, it is difficult to increase the mass of the head 1. In this aspect, by increasing the distance from the center of gravity (r) to the extent allowed by the relation to the mass (m) of the head 1, the moment of inertia is increased. In particular, when the lie angle of a metal hollow golf club head is set to 60°, the moment of inertia about the axial line centered on the plumb line passing through the center of gravity of the head 1 is increased. Below, the head construction by which this was achieved is explained.

FIG. 5 is a cross-sectional view along X-X in FIG. 4. The cross-sectional view is in the same direction as FIG. 3, passing through the center of gravity 8. In the figure, the symbol B is the distance from the center of gravity 8 to the hosel portion 7, that is, the center-of-gravity distance. The mass and shape of the hosel portion 7 in effect cannot be changed, due to the circumstances of mounting of the prescribed shaft A. Hence in order to increase the moment of inertia, it is effective to increase the distance at which a mass is positioned from the center of gravity 8, that is, the separation distance C, and to increase the mass. Hence in this Aspect 1, by changing the placement position the separation distance C is increased. As explained above, the crown portion 2 is formed by pressing of a sheet-shape titanium alloy member; this press-machined member is formed into an even thinner shape. In this crown portion 2, the thickness in the area D on the periphery of the center of gravity above the center of gravity 8 is made thin. That is, when the lie angle of the head 1 is set to 60°, this center-of-gravity periphery area D is the area on the reverse face of the crown portion 2 centered on the plumb line (not shown) passing through the center of gravity 8 of the head 1.

The machining method to reduce the thickness of this center-of-gravity periphery area D is, in this example, chemical etching treatment. This chemical etching treatment is well-known, and a detailed explanation is omitted. Chemical processing is used to reduce the thickness in the center-of-gravity periphery area D of the crown portion 2, to further reduce the thickness of the crown portion 2. As the machining method used to reduce the thickness of the crown portion 2, cutting and coining by press machining are also possible to some degree; but use of such machining methods is limited by the machining hardness of the materials and by various machining-related constraints, and thicknesses beyond a certain limit cannot be attained. Through this chemical etching treatment, the thickness of the center-of-gravity periphery area D can be reduced to the desired value. By this means, titanium alloy alone can be used to maintain strength, rather than opening a hole in the crown portion and covering with a fiber-reinforced resin instead of a metal material in a composite configuration, as in the prior art.

Cutting-away of the center-of-gravity periphery area D of the crown portion 2 results in reduced mass of the crown portion 2, and consequently the mass at sites relatively distant from the center-of-gravity periphery D is increased. Further, as shown in FIG. 5 and FIG. 6, the portion in which cutting-away is avoided is the side of the toe portion 5 opposed to the hosel portion 7 from the center of gravity 8. More rigorously, the position of the boundary portion on the side of the toe portion 5 and back portion 10 (see FIG. 2) is preferable. This portion can be made distant from the center of gravity 8, and has the greatest effect in increasing the moment of inertia. Moreover, it is preferable that further mass be placed at this portion, that is, at the end position at the separation distance C.

In this Aspect 1, as explained above, this is accomplished by reducing the mass of the center-of-gravity periphery area D and leaving a thickness 2 a at the end position at the separation distance C in the crown portion 2. This results in the occurrence of a relative difference in mass between the center-of-gravity periphery area D and the end portion at the separation distance C. As a result, if the overall mass of the head 1 does not change, this is equivalent to providing a mass at the position of the end portion at the separation distance C. Moreover, the separation distance C was increased to the extent possible within the limitations of the overall mass. This was accomplished by enlarging the grown portion 2 b on the side of the toe portion 5, and forming a substantially rectangular shape. The shape approximates a so-called square wood. This result was accomplished by reducing the thickness of the center-of-gravity periphery area D within the limitation of the overall mass.

Next, an excess mass 3 a is provided at the sole end portion position at separation distance C in the sole portion 3. This is a means to directly increase the mass, but in contrast with masses provided with the aim of more effective striking as in the prior art, the position of placement of the mass is limited, and the mass is provided at the site farthest removed from the position of the center of gravity 8, that is, at the site positioned at the separation distance C. The mass increase due to this excess mass 3 a is within the limitation on overall mass, but to the extent that the mass of the center-of-gravity periphery D of the crown portion 2 is cut away, this mass can be added, so that greater mass increase is possible than in the prior art. For example, whereas in the construction of a conventional head an added mass was approximately 10 g, in this aspect 1, addition of a mass of 20 to 25 g is possible. An added weight does not only take the form of provision of a separate body, but also includes weight added by increasing the thickness of a plate member itself.

In order to make the moment of inertia of the golf club head greater than 5000 g-cm², it is necessary to make the position of center of gravity optimal under the restricted condition(s) of the golf club head. This is brought about, for example, by securing a surplus mass, by thinning a portion in an appropriate position and imparting the surplus mass to the portion, at an appropriate position. As the total mass of the club head is limited to approximately 210 g, the surplus mass for weights is found to be approximately 20 to 25 g, which is to be imparted to an appropriate portion(s) in appropriate position(s). Here, for example, a thick portion with thickness of approximately 1.0 mm or so is necessitated in order to prevent deformation of the welded portions and/or to preserve strength against impact.

Considered from these, for example, with the constituting material of the club head being titanium alloy of a relatively light specific gravity, a surplus mass of more than approximately 10 g is removed from each of the crown portion and the sole portion, which results in removal of a total mass of more than approximately 20 g. This necessitates, for example, thinning of the wall thickness by more than 0.2 mm for areas of more than 60% in both of the crown portion and the sole portion. In this example, it is preferable to make the area of the thinned portion larger in the crown portion than in the sole portion, in respect of strength of the whole club head or in making the club head of a low center of gravity.

In this way, in this Aspect 1, a portion of the crown portion 2 is removed by chemical etching, and a mass 3 a is provided in the sole portion 3, to increase the value of the mass m to the extent possible. As described above, with respect to the numerical value of the distance r, by forming the side of the extended crown 3 b in particular into an enlarged rectangular shape, the separation distance C of the mass 3 a from the center of gravity 8 can be increased. This separation distance from the center of gravity 8 is indicated by C in FIG. 5; in FIG. 2, the extended crown portion 3 b is in a position on the side opposite the substantially diagonally line opposing the center-of-gravity distance B on the side of the hosel portion 7, and has an enlarged shape compared with the extended crown portion 3 b in the position of the prior art. As a result, through the multiplied effect of the increased numerical values of the distance (r) from the center of gravity 8 and of the mass (m) of the head 1, a moment of inertia of 5000 to 6000 g-cm² is achieved in a head with an increased volume of 470 cm³.

Aspect 2

FIG. 6 is a plane view showing Aspect 2 of the invention. In this example, relative to the crown portion 2, the end portion 3 b of the sole portion 3 on the side of the toe portion 5 and back portion 10 is enlarged, and a mass 3 a is positioned. Other portions of this head 1 are effectively the same as in Aspect 1, and explanations are omitted. In this Aspect 2, the numerical value of the distance r equivalent to the separation distance C is further increased, so that the moment of inertia can be further increased. Moreover, when examining the moment of inertia of a metal hollow golf club head, the lie angle is set to 60°. The lie angle is the angle bade by the face of the head in contact with the ground and the shaft; in the case of a metal hollow golf club head, even when the head is enlarged with increased volume and large moment of inertia, experiments have indicated that a lie angle of approximately 60° is most satisfactory.

Making the moment of inertia large in this way means that, even when contact with the ball deviates from the center, or in other words is on the side of the toe portion 5 from the “sweet” area 9 of the face portion 4, or in an extreme case the ball is mis-struck, because vibration of the head 1 does not readily occur there is little bending of the hit ball compared with the prior art, and the direction of the hit ball is stable compared with a case of a smaller moment of inertia.

Aspect 3

FIG. 7 to FIG. 9 show Aspect 3 of a driver club head; FIG. 7 is a plane view of Aspect 3 of a driver club head, FIG. 8 is a front view of Aspect 3 of a driver club head, and FIG. 9 is a side view of Aspect 3 of a driver club head. The external shape of this head 1 is substantially quadrilateral as seen in plane view, with the angle in one corner formed into an arc, as represented characteristically in the plane view of FIG. 7. The metal hollow driver club head 1 of this Aspect 3 is created entirely from titanium alloy sheet. The body members comprised by the head 1 of this Aspect 3 are the four members which are the face portion 4, sole portion 3, crown portion 2, and hosel portion 7. The crown portion 2 comprises titanium alloy sheet (specific gravity 4.51) of uniform thickness 0.5 mm. The sole portion 3 comprises titanium alloy sheet (specific gravity 4.51) of uniform thickness 0.75 mm. Similarly, the hosel portion 7 and weight 3 a comprise titanium alloy (specific gravity 4.51).

The face portion 4 is entirely formed from titanium alloy sheet (specific gravity 4.42); the thickness is different in different portions. In the ellipse-shaped center portion 4 a, the thickness is 3.1 mm. In the outer portion 4 b on the outside of the center portion 4 a, the thickness is 2.3 mm. In this way, the thickness differs in different portions of the face portion 4 in order to cause the characteristic time (CT value) of the restitution coefficient of the face portion 4 to be the stipulated 257 μs or less (including a tolerance of 18 μs), and so that the mass of the face portion 4 is not increased. The flange 4 c on the outer periphery of the face portion 4 is of 1.3 mm thick titanium alloy sheet (specific gravity 4.42) with constant width. The flange 4 c is also placed at the positions of the crown portion 2 and sole portion 3. This flange 4 c supports the face portion 4 on the outer periphery, and in addition functions to link the crown portion 2 and sole portion 3, which are thin, and the face portion 4. The construction of the hosel 7 has a generally employed shape, and the shape is not special, so that a detailed explanation is omitted.

Moment of Inertia MOI

On both sides of the back portion 10 is placed a weight 3 a. As shown in FIG. 7, more of the weight 3 a is placed at the corner portions on both ends. This is in order to increase the distance from the center of gravity 8 and increase the moment of inertia MOI. As shown in FIG. 8, the distance from the end of the toe portion 5 of the head 1 to a point in the heel portion 5 at a height of 22.23 mm (0.875 inches) from the bottom face of the sole portion 3 is called the “toe-heel length X”. As shown in FIG. 9, the length from the end of the face portion 4 to the end of the back 10 is called the “head width Y”. Given the construction indicated for this Aspect 3, the moment of inertia MOI about the plumb line passing through the center of gravity 8 was calculated. The method of calculation was as follows. Taking as basic the head 1 of this Aspect 3, the external dimensions were systematically varied, and the moment of inertia MOI calculated for each case.

FIG. 10 arranges the external shapes of heads 1 for which the moment of inertia MOI was calculated. Heads 1 arranged in this way have increasingly larger “toe-heel lengths X” in moving rightward along the horizontal axis (in the figure) (87 to 127 mm), and have increasingly larger “head widths Y” in moving upward along the vertical axis (in the figure) (86 mm to 126 mm). These heads 1 have a hosel portion 7 with the same shape and dimensions as in Aspect 3; the flange width dimension of the outer periphery 4 c of the face portion 4 is also the same, and as shown in FIG. 10, only the head width Y and toe-heel length X were varied. However, rules for golf equipment stipulate that the “toe-heel length X” must not be longer than the “head width Y”. Hence heads 1 in the upper-left in FIG. 10 cannot be used, and so calculations were omitted.

The heads 1 shown in the first column (right-hand column) in FIG. 10 have a “toe-heel length X” fixed at 127 mm, and “head widths Y” of 126 mm, 116 mm, 106 mm, 96 mm, and 86 mm. The moment of inertia MOI was calculated for each of these as described below. The head width Y (mm), head volume (cm³), moment of inertia MOI (g-cm²), and mass of the weight 3 a (g), appear in Table 1. The overall masses for heads 1 were 205 g in each case. Hence as shown in Table 1, the volumes, and the masses of weights differed for each of the heads 1. The moment of inertia about the axial line (lateral) centered on the plumb line passing through the center of gravity of the head 1, which has been an object of this invention, was in the range 5000 to 6000 g-cm². As is seen from the data of Table 1, for a “head width Y” of 126 mm, 116 mm, and 106 mm, a moment of inertia of approximately not less than 5000 g-cm², which was an object of this invention, was exceeded.

TABLE 1 Head Volume Moment of inertia Weight mass Width(mm) (cm³) (g-cm²) (g) 126 460.8 5,900 43.91 116 424.6 5,424 51.23 106 388.3 4,967 58.40 96 352.0 4,550 65.68 86 315.7 4,181 72.91 Here, the “toe-heel length X” is 127 mm, and the head mass is 205 g.

Similarly, the heads 1 in the second column from the right in FIG. 10 have the “toe-heel length X” fixed at 117 mm, and “head widths Y” of 126 mm, 116 mm, 106 mm, 96 mm, and 86 mm; the moments of inertia MOI for each of these were calculated. The data appears in Table 2. As is seen from this Table 2, when the “head width Y” is 126 mm or 116 mm, the desired moment of inertia of 5000 g-cm² or higher is achieved.

TABLE 2 Head Volume Moment of inertia Weight mass Width(mm) (cm³) (g-cm²) (g) 126 424.6 5,581 53.05 116 391.1 5,046 60.01 106 357.7 4,537 66.66 96 324.3 4,074 73.48 86 290.8 3,664 80.13 Here, the “toe-heel length X” is 117 mm, and the head mass is 205 g.

FIG. 11 shows the relation between head width and moment of inertia, when the toe-heel length is held constant. That is, FIG. 11 plots the results of Tables 1 and 2 with the head width Y along the horizontal axis and the moment of inertia MOI along the vertical axis; the moment of inertia MOI is plotted along the vertical axis with the “toe-heel length X” dimension fixed. This FIG. 11 plots the moments of inertia MOI of the heads 1 with the construction of Aspect 3, holding the “toe-heel length X” constant. From this data, heads with a moment of inertia MI exceeding 5000 g-cm², which is an object of this invention, are approximately three in number when the “head width Y” is 126 mm, and are approximately two in number when the “head width Y” is 116 mm.

Probabilistically, the moment of inertia is near 5000 g-cm² when the “toe-heel length X” is 127 mm, and so this data is adopted as an approximating equation. When in FIG. 11 each of the points of the moment of inertia for which the “toe-heel length X” is 127 mm are connected, the resulting figure is a quadratic curve. An approximating equation for this quadratic curve is equation (3) below, and so when the “toe-heel length X” is fixed, this approximating equation is adopted in the vicinity of 5000 g-cm².

MOI=0.181Y ²+4.191Y+2437.7   (3)

Similarly, heads 1 in the first row from the top in FIG. 10 have the “head width Y” fixed at 126 mm, and the “toe-heel length X” at 127 mm and 117 mm. However, a “toe-heel length X” of 117 mm violates the rules, and so was removed from consideration. Similarly, heads appearing in the second row in FIG. 10 have the “head width Y” fixed at 116 mm, and a “toe-heel length X” of 127 mm and 117 mm. Similarly, heads appearing in the third row in FIG. 10 have the “head width Y” fixed at 106 mm, and a “toe-heel length X” of 127 mm, 117 mm, and 107 mm. Similarly, heads appearing in the fourth row of FIG. 10 have the “head width Y” fixed at 96 mm, and a “toe-heel length X” of 127 mm, 117 mm, 107 mm, and 97 mm. Similarly, heads appearing in the fifth row of FIG. 10 have the “head width Y” fixed at 86 mm, and a “toe-heel length X” of 127 mm, 117 mm, 107 mm, 97 mm, and 87 mm. In this way, moments of inertia MOI of the heads were calculated with the “head width Y” dimension fixed.

FIG. 12 shows the relation between the toe-heel length and the moment of inertia when the head width is held constant. That is, FIG. 12 plots the “toe-heel length X” along the horizontal axis and the moment of inertia MOI along the vertical axis with the “head width Y” dimension fixed. From this data, heads with a moment of inertia exceeding 5000 g-cm², which is an object of this invention, are only heads with the head width Y fixed at 106 mm. The line connecting these points is a straight line, an approximating equation for which is given by equation (4); this equation (4) is adopted as an approximating equation in the vicinity of 5000 g-cm².

MOI=37.84X+618.3   (4)

In the vicinity of a moment of inertia of 5000 g-cm² or higher, as the relation between the toe-heel length (X) and head width (Y), these two approximating equations (3) and (4) are combined to obtain the following approximating equation (5) in the vicinity of 5000 g-cm².

MOI=(0.181Y ²+4.191Y+2437.7)×((37.84X+618.3)/5424)   (5)

This equation (5) is a product of the approximating equation (3) and the approximating equation (4); by taking the product of the two approximating equations, a relation is derived between the moment of inertia in the vicinity of 5000 g-cm², the head length X, and the head width Y. Here the numerical value “5424” is the moment of inertia when the “toe-heel length X” in approximating equation (4) is 127 mm. Approximating equation (5) is the product of approximating equation (3) and approximating equation (4), and so is divided by the moment of inertia calculated using approximating equation (3) in the vicinity of 5000 g-cm². By this means, the value of the moment of inertia of approximating equation (5) is corrected.

As is understood from the above explanation, the numerical values in approximating equation (5) are intrinsic numerical values arising from the shape, construction, materials, masses, and similar of the head specific to this Aspect 3, and so these values can be replaced with constants. That is, the approximating equation (5) can be represented as the following general equation.

MOI=(aY ² +bY+c)×(dX+e)/f   (1)

Here X is the length from the heel portion to the toe portion, Y is the length from the face portion to the rear surface, and a, b, c, d, e, and f are constants.

FIG. 13 is a graph showing the relation between the toe-heel length and the head width for different moments of inertia. FIG. 13 is a general summary of the above explanation; here the horizontal axis indicates X (toe-heel length) and the vertical axis indicates Y (head width). In FIG. 12, values above the diagonal line have a head width greater than the toe-heel length, and violate the rules, and so describe heads which cannot be commercialized. The area below the diagonal line describes heads which comply with the rules, and represents heads having a lateral moment of inertia of 5000 g-cm² or higher, an object of this invention, which have been manufactured and commercialized.

The above-described approximating equation (5) was calculated for each of the moments of inertia 5000, 5200, 5400, 5600, 5800, and 5900 g-cm². From FIG. 13, if a golf club head having a moment of inertia in the range 5000 to 5900 g-cm², which is an object of this invention, is to be obtained, the toe-heel length and head width of a head with the shape of that of Aspect 3 can be determined. Further, as a result, the weight magnitude can also be determined.

Center of Gravity Position

Next, differences in the moment of inertia with the center of gravity position of the head of Aspect 3 are explained. In FIG. 14A to 14E are examples in which the position of placement of a weight 3 a in the head 1 is varied. The specifications and thicknesses of the crown portion 2, sole portion 3 and face portion 4 are as described above. Further, when the specifications of this head 1 include a “toe-heel length X” of 127 mm, “head width Y” of 126 mm, volume of 460 cm³, and head mass of 205 g, the weight mass is 43.9 g. FIG. 15 shows the relation between moment of inertia and center of gravity position for heads with these specifications. The horizontal axis u in FIG. 15 indicates the u-direction center of gravity position, in the toe-heel length direction. The vertical axis v indicates the v-direction center of gravity position, in the head width direction. The origin (O) of the horizontal axis u is the center position of 127 mm. The origin (O) of the vertical axis v is the surface of the face portion 4. Hence FIG. 15 is equivalent to a plane view of the head 1.

The curve in FIG. 15 for a moment of inertia of 5000 g-cm² is the result of using the above-described approximating equation (5) to plot the center of gravity in the vicinity of a moment of inertia of 5000 g-cm². This curve is the result of fixing the overall mass of the head 1 at 205 g, the mass of weight 3 a at 43.91 g, the toe-heel length at 127 mm, the head width at 126 mm, and the volume at 460 cm³, so that the moment of inertia is 5000 g-cm², and representing the center of gravity position when the position of the weight 3 a is varied as shown in FIG. 14. The mass of the weight 3 a is determined when the overall mass of the head 1 is 205 g and the volume is constant at 460 cm³. At this time, the center of gravity position of head 1 is changing, and the moment of inertia is changing, when the position of the weight 3 a is varied as in FIG. 13, the maximum moment of inertia area, and the area in which the moment of inertia is 5000 g-cm², can also be calculated. This maximum moment of inertia area and area resulting in a moment of inertia of 5000 g-cm² are indicated by line segments.

The maximum moment of inertia curve and the 5000 g-cm² moment of inertia curve are represented by the following approximating equations (6) and (7).

v=−0.0668u ²+0.1318u+56.66   (6)

v=−0.1558u ²+0.6363u+41.53   (7)

Hence one condition to obtain a head with a moment of inertia of 5000 g-cm² or above is that the center of gravity position be set in the area enclosed between these approximating equations (6) and (7). The position and size of the “sweet” area of the face portion 4 are also related to this center of gravity position, and so are important.

As is understood from the above explanation, each of the numerical values of the approximating equations (6) and (7) are intrinsic numerical values arising from the shape, construction, materials, masses, and similar of the head specific to this Aspect, and so these values can be replaced with constants. That is, the approximating equation (5) can be represented as the following general equations.

v=gu ² +hu ² +i   (2)

v=ju ² +ku+1   (3)

Here, u is the position from the heel portion in the toe portion direction, v is the position from the face surface in the rear face direction, and g, h, j, and k are constants; the origin of u is taken to be the center of the length from the heel portion to the toe portion, and the origin of v is taken to be the face surface.

Other Aspects

As explained in detail above, aspects of the invention are configured as described above; but of course this invention is not limited to these aspects. For example, the above-described crown has different thicknesses in substantially the center portion and in the outer peripheral portion, but the entire crown portion may be of a smaller thickness than other body portions. Similarly, the thickness of the center area of the sole portion may be reduced, or the thickness of the entire sole portion may be made thinner than other body portions. Also, the materials comprised by the head are in essence all metal; but very small amounts of other materials can be used in some portions. Moreover, the numerical values stipulated by rules include tolerances, and of course even when related numerical values fluctuate within the ranges of tolerances they remain within the technical scope of this invention. 

What is claimed is:
 1. A golf club, comprising: a face portion positioned on the front surface of a metal hollow golf club head and having a striking face to strike a golf ball; a crown portion forming an upper surface of the club, and a sole portion forming a lower surface of the club; wherein a mass of the metal hollow golf club head is 210 g or less, characteristic time (CT value) of the metal hollow golf club head, relating to a restitution characteristic, is 257 μs or less, the volume of the metal hollow golf club head is 400 to 470 cm³ when a lie angle of the metal hollow golf club head is 60 degrees, the metal is made of titanium alloy entirely and formed through welding to be an integrated body the crown portion and sole portion constituting the body has an area of thick portion and an area of thinned portion, the area of the thinned portion amounts to more than 60% in both of the crown portion and the sole portion, and the thickness difference between the thinned portion and the thick portion is more than 0.2 mm and the removed mass amounts to more than 10 g in both of the crown portion and the sole portion, a weight is located and fixed at a position of a rotation radius most distant from the plumb line and at toe-side and back-side sites of the crown portion and/or the sole portion, the metal hollow golf club head has: a length (X) from a heel portion to a toe portion longer than a length (Y) from a face portion to a rear surface; a length from a heel to a toe of 127 mm (5 inches) or less; a length from a sole to a crown of 71.12 mm (2.8 inches) or less; and a moment of inertia (MOI) calculated using the following approximating equation (1): MOI=(aY ² +bY+c)*(dX+e)/f   (1) where, in the approximating equation (1), the second order equation of Y is determined by approximating the MOI value with a second order equation of variable Y as a head width under a constant value X as a toe-heel length and the constants a, b and c are coefficients of the terms in the second order equation respectively, the first order equation of X is determined by approximating the MOI value with a first order equation of a variable X as a toe-heel length under a constant value Y as a head width and d and e are constants of terms in the first order equation respectively, and f is a value of MOI with a certain selected value of X satisfying the required condition; a position of the center of gravity (u, v), as seen from the plumb direction, exists within a range encompassed by the following two approximating equations: v=gu ² +hu+i   (2) v=ju ² +ku+1   (3) where u is a position of center of gravity in the direction from the heel portion to the toe portion, v is a position of center of gravity in the direction from the face portion to the rear surface, u has an origin at the center of the distance from the heel portion to the toe portion and v has an origin in the face surface; and the approximating equation (2) is determined by approximating the relation between u and v with a second order equation when MOI is the maximum, g, h and i are constants of terms of the equation respectively, and the approximating equation (3) is determined by approximating the relation between u and v with a second order equation when MI is 5000 gcm², j, k and l are constants of terms of the equation respectively, and the moment of inertia about an axial line, which is the plumb line passing through the center of gravity of the metal hollow golf club head, is in a range of 5000 to 6000 g-cm².
 2. The golf club according to claim 1, wherein the metal hollow golf club head is formed by welding to join the face portion, the sole portion, the crown portion and a hosel portion to which the shaft is connected.
 3. The golf club according to claim 1, wherein the weight is 20 g or more.
 4. The golf club according to claim 1, wherein toe-side and back-side sites of the sole portion are formed in shapes protruding outward relative to the crown portion, and the weight is positioned in this protruding sole portion.
 5. The golf club according to claim 1, wherein the area of the thinned portion is larger in the crown portion than in the sole portion.
 6. The golf club according to claim 1, wherein the thinned portions in the crown portion and the sole portion are formed by chemical etching. 